X-ray crystallographic techniques, using film methods of data collection, will be used to determine the tertiary structure and catalytic mechanism of the enzyme lactose synthetase. The principal objectives of this research are: (l) To determine the high resolution x-ray crytal structure of alpha-lactalbumin from either human or cow's milk. (2) To crystallize and determine the x-ray crystal structure of galactosyl transferase, the A protein component of lactose synthetase. (3) To crystallize and determine the x-ray crystal structure of stable complex of alpha-lactalbumin with galactosyl transferase. (4) To compare the conformations of the homologous proteins alpha-lactalbumin and hen egg white lysozyme. (5) To investigate the unique role of alpha-lactalbumin as a specifier protein in the lactose synthetase reaction in terms of protein-protein interaction and the binding of substrate analogues. Galactosyl transferase, the membrane bound component of lactose synthetase has recently been identified as a marker protein present in the serum of patients with certain types of malignant tumor. Structural studies of the metabolic control of this enzyme at the molecular level may have implications for the advancement of cancer research.